Among materials which change in color when exposed to light is a photochromic material. The photochromic substance has the property that its structure reversibly changes according to the presence or absence of ultraviolet radiation to change in absorption spectrum. This property results from the nature that when an isomer is irradiated with light of a particular wavelength, the single chemical substance, under the action of light, reversibly produces an isomer having a different absorption spectrum. The resulting different isomer is restored to the color of the original isomer by heat or light of a different wavelength.
Photochromic spectacles utilize the above nature of the photochromic material for lenses. In an outdoor environment exposed to light including ultraviolet radiation, such as sunlight, the lenses are rapidly colored to function as sunglasses. In an indoor environment without exposure to light, the lenses fade to function as ordinary transparent spectacles.
Known methods for producing a lens having photochromic property include a method of impregnating the surface of a non-photochromic lens with a photochromic coating fluid; a method which comprises dissolving a photochromic coating fluid in a monomer, and polymerizing the solution to obtain a photochromic lens directly; and a method comprising providing a layer having photochromic property on the surface of a lens with the use of a coating agent containing a photochromic compound (namely, a coating method). An apparatus for forming a layer having photochromic property on a lens by the coating method is disclosed.
In the above-described coating method, when the photochromic coating fluid is supplied onto the lens, the photochromic fluid is ejected from the nozzle of a vessel containing the photochromic fluid to the surface of the lens to form a coating layer on the surface of the lens.
However, the photochromic fluid has viscosity, so that if air is entrained in the fluid, air bubbles are formed in the resulting coating layer. The nozzle of the vessel is provided with a check valve for preventing fluid dripping. In this case, air may be incorporated in the nozzle at the start of operation, thus requiring that air in the nozzle be discharged outside before start of the operation. For this purpose, air purge of the nozzle is performed using the coating fluid. This poses the problem of a great expenditure, because the photochromic fluid is expensive.
The present invention has been accomplished in the light of such circumstances. It is an object of the invention to provide a removal method for entrained air in a coating nozzle, and a coating apparatus, the removal method and the coating apparatus unable to form an uniform photochromic coating film without voids and thickness unevenness.
In connection with the present invention, technologies such as the technology described in Japanese Unexamined Patent Publication No. 2000-334369 concerning with an apparatus which can coat a plurality of lenses continuously, the technology described in Japanese Unexamined Patent Publication No. 2005-013873 concerning with an apparatus equipped with an auxiliary mechanism for spreading a coating fluid on a lens with the use of a flexible film, and the technology described in Japanese Unexamined Patent Publication No. 2005-218994 concerning with an apparatus which adjusts the rotational state of a lens according to the shape of the lens or the viscosity of a coating fluid are available.